Pollution and pollution control have become increasingly more important in recent years as a result of an increased awareness and understanding of the adverse impact that pollution can have on the environment. Environmental pollution is but one consequence of industrialization and, in addition to causing a continuing deterioration in the quality of the natural environment, represents both real and potential health hazard to mankind.
Water being the essential resource that it is, industrial pollution of our water supplies has become one of our greatest concerns. Any use of water nearly always results in a decrease in the quality of the water through the addition of pollutants. These pollutants can comprise dissolved inorganic and organic chemicals, inorganic and organic particulate material and absorbed gases. Several methods have been developed and are available for removing such pollutants. These include such processes as gravity sedimentation, flotation, filtration, ion exchange, activated adsorption, reverse osmosis, electrodialysis, distillation and chemical precipitation. However, as successful as these processes may be, there exist several soluble inorganic pollutants that generally are not removable by the above enumerated methods, an example of which is selenium.
Stringent standards for the maximum level of pollutants in water to be used for drinking or to be released into groundwater systems have been or are being promulgated by both federal and state environmental agencies. For example, the current allowable maximum concentration for selenium in drinking water set by federal standards is 0.01 milligrams per liter. On the other hand, the State of New Mexico has set a selenium standard of 0.05 milligrams per liter for water discharged into the groundwater systems of that state.
An increase in pollutant concentration is, in itself, not significant, provided that methods are available for reducing the pollutant concentration to acceptable levels. One method employed to remove or substantially reduce the concentration of soluble inorganic pollutants such as heavy metals in water is chemical precipitation of the metals as either their oxides or their hydroxides. This precipitation generally is affected by the addition of lime, alum or an iron salt to the water at an appropriate pH.
Other treatment methods, such as ion exchange, reverse osmosis, electrodialysis or distillation, also can be effective in removing various pollutants. However, these methods are considerably more expensive and generally narrower in applicable scope than is desirable for the treatment of great volumes of water as is necessitated in many industrial operations.
It is known that selenium ions can be removed from aqueous systems employing chemical precipitation if the selenium is present in the selenite or Se(IV) oxidation state. Generally, such precipitation methods comprise treating the selenite-containing aqueous system with an iron salt, such as ferric or ferrous sulfate, chloride or hydroxide, or with aluminum or zinc in some appropriate form such as a power, granules or the like. However, such chemical precipitation methods provide only very limited removal of selenium when it is present in the selenate or Se(IV) oxidation state. Therefore, when present in the selenate oxidation state, selenium removal generally is affected by either ion exchange or reverse osmosis. See Thomas J. Sorg and Gary S. Logsdon, "Removal of Selenium From Drinking Water", Proceedings of the Symposium on Selenium-Tellurium in the Environment, pp. 114-128, May 11-13 (1976).
A recent U.S. Patent, U.S. Pat. No. 4,405,464, provides a method by which selenium, as the selenate, is chemically precipitated from the aqueous system containing the same using metallic iron. This latter process also is disclosed as being capable of removing a substantial portion of any selenium in its selenite oxidation state. This capability, alone, renders the process described in U.S. Pat. No. 4,405,464 economically more attractive, than either the ion exchange or reverse osmosis methods which have been proposed or which are currently in use.